Fluorocarbon propellants have been in common use for aerosol products, such as aerosol hair sprays, for many years. Since the propellant on release from the container forms a gas which enters the atmosphere, there have been increasing objections to the use of such propellants on the grounds of atmospheric pollution. Moreover, it has been recognized that carbon dioxide is a safe non-polluting propellant. Considerable research and development work has therefore been devoted to the substitution of carbon dioxide for the fluorocarbon propellants. Some success has been obtained with through-the-valve filling equipment. In this procedure, the liquid fill is introduced into the containers, the cap units are applied and crimped, and the CO.sub.2 gas is charged through the valves of the attached cap units.
The foregoing procedure can be used with present commercial through-the-valve filling machines. However, the majority of aerosol filling machines in current commercial use are of the undercap-type. With these machines, the fluorocarbon propellant in liquid form is charged to the container with the cap assembly sitting loosely on top of the container. The liquid propellant flows into the container between the cap flange and the container sealing bead. Immediately following such propellant charging, the caps are crimped and sealed.
Heretofore, it has been believed that the undercap-type filling machines could not be used successfully for charging aerosol containers with carbon dioxide gas, except by a special auxilliary procedure in which a premix of the CO.sub.2 propellant and the liquid fill is formed, and then filled as a liquid. This requires a system including a large gas-liquid impregnator unit, and other equipment for transferring the liquid premix to the containers.
From the standpoint of present commercial operations and equipment, what has been needed is a method which permits the standard undercap filling machines to be used in the same manner as they have been heretofore used, except that CO.sub.2 as a gas is charged to the containers instead of the liquid fluorocarbon propellant. However, when this was tried with CO.sub.2 gas at sufficiently high pressure to function effectively as a propellant many difficulties were encountered. When the can pressure did not reduce rapidly enough by the dissolving of the CO.sub.2 gas in the liquid, the cans would rupture. Also, the crimping and sealing was erratic. Containers were improperly sealed because the cap was not fully seated on and crimped to the sealing bead. Where the cap was canted, on crimping, package rejects occured because of present or potential leakage. Such attempts were therefore discouraging.
As far as is known, the present invention for the first time provides a practical commercially feasible method for undercap filling or aerosol containers with carbon dioxide gas. The method can be carried out on standard undercap filling machines with only a minor modification, in the nature of an adjustment. Present users of undercap filling machines can therefore immediately and easily convert to use of CO.sub.2 gas propellant, thereby eliminating the pollution hazard of fluorocarbon propellants.